This invention relates to a piston probes which are used in testing microscopic circuits such as those which are in semiconductor chips and multichip interconnect modules.
During the fabrication of semiconductor chips, it is necessary that the bare multichip interconnect module (such as that described in U.S Pat. No. 4,721,831) be probed and tested before it receives any integrated circuit chips. Such separate testing avoids expensive reworks and/or scrapping of the unit which the permanently joined chips and module form.
In the prior art, the probing of various contacts on the chips and the interconnect modules was performed by what is commonly called xe2x80x9ca bed of nailsxe2x80x9d. This bed of nails is comprised of a plurality of thin wires which are held in a cantilevered fashion such that their ends form a pattern which corresponds to the contacts that are to be probed. Thus the ends of the wires constitute probe tips. Electrical connections between these probe tips and the contacts on the chips or interconnect module are made by mechanically urging them together until the wires deflect slightly.
A problem, however, with these bed of nails probes is that the maximum density of the probe tips is too limited. State of the art probes can only be arranged in patterns of about 10 mils (254 micrometers) on center. By comparison, the transistors and their interconnect lines within an integrated circuit chip and interconnect module can have feature sizes of one micrometer or less.
Also, another problem with the bed of nails probes is that their wires are very fragile and are easily bent. Thus, as a practical matter, the total number of probes in a bed of nails is limited to less than 1,000. By comparison, the total number of contacts on a multichip interconnect module can be several thousand.
Still another problem with the bed of nails probe is that the force between a probe tip and its contact is determined by many factors which are difficult to control. For example, a probe wire may be slightly bent, or the contacts on an interconnect module may be at slightly different heights. And, an excessive probe tip force can damage the corresponding contact, whereas small probe tip force can yield an open circuit.
Accordingly, a primary object of the invention is to provide an improved electro-mechanical module test apparatus which overcomes the above problems.
Testing of electronic packaging or modules has become increasingly difficult because of the fragile nature of the semiconductor chips. During testing the chips are subject to forces that can damage the chips especially when forces are applied unevenly.
During electrical test of the complete module, a great amount of pressure is required at module contact area to assure good electrical connection to the tester""s contacting pins. In the case of a module with an array, such as a PBGA, the forces exerted upon the module may be extremely high if the array is of a small spacing between connections (e.g. 1.0-mm grid or less).
Presently, these forces are exerted upon the module either upon the xe2x80x9ccapxe2x80x9d or xe2x80x9clidxe2x80x9d in the case of a xe2x80x9clidded modulexe2x80x9d, or around the peripheral zone in the case of a lidless module. Direct pressure of the level required may not be fully placed upon the mounted silicon chip, as this would damage the chip.
In the instance of a lidless module wherein the peripheral zone is limited due to capacitors mounted on the top of the carrier and where the carrier is made of thin organic based material, a peripherally applied force in a narrow band may exert forces which result in excessive bending stresses in the center of the carrier where the area array of contacts is fully populated.
In the present invention a novel piston is used to conform to the topology of the overall surface area of the module. The piston is tailored by the diameter and length of pressure pins to apply an equal conformed pressure on the surface which will force the chip into electrical and mechanical contact with various testing apparatus such as those which interact with:
thin carrier has full array of contacts upon the bottom, limited area on top for contacting; or
where contact pressure with peripheral (window frame) technique, are unable to place large load on chip; or
where pressure results in bending of carrier.
The piston of the present invention utilizes an array of pressure pins protruding from the bottom that contact the module below. The top of pins have a customized with the head of the nails arrangement that have a predetermined length and diameter the head of the pins are subjected to equal pressure, but each pin will provide a different force (function of pin head area). The module surface will get pressure, but customized to be at an acceptable level. Net result: less bending of carrier and acceptable test contact method.
In operation, the piston frame may be fitted with a cover which overlies the heads of the pins to thereby form an enclosed chamber. The chamber may be pressurized to force the movable pins downward against the semiconductor chip or interconnect module which is to be tested. Also, a port is provided in the cover so that a gas can be input to the chamber to expand the elastic membrane and thereby force the pins against the contacts of the circuit being tested.